Frequency converter

ABSTRACT

According to this invention, a frequency converter comprises a frequency mixer (11) formed in a semiconductor integrated circuit (10), an impedance conversion/signal amplitude limit differential amplifier (12) formed in the semiconductor integrated circuit and connected to the output of the frequency mixer, a first frequency mixing output terminal (14) connected to an output terminal of the frequency mixer and serving as an external terminal of the semiconductor integrated circuit, and a second frequency mixing output terminal (15) connected to an output terminal of the differential amplifier and serving as an external terminal of the semiconductor integrated circuit.

TECHNICAL FIELD

The present invention relates to a frequency converter used forconverting, e.g., an FM reception signal of an FM receiver into anintermediate-frequency signal.

BACKGROUND ART

Conventionally, a frequency converter used for converting, e.g., an FMreception signal of an FM receiver into an intermediate-frequencysignal, is arranged as shown in FIGS. 1 to 3.

In a circuit in FIG. 1, reference numeral 31 denotes a frequency mixerfor mixing an FM reception signal (high-frequency signal) and a localoscillation signal. The mixer 31 is formed in a bipolar integratedcircuit, 30. Reference numeral 32 denotes an intermediate-frequencytransformer for extracting an intermediate-frequency component of 10.7MHz from an output of the frequency mixer 31. The transformer 32 isconnected between a Vcc power source terminal 34 and a frequency mixingoutput terminal 35 outside the integrated circuit 35. Reference numeral33 denotes a ceramic filter (center frequency is 10.7 MHz), forobtaining desired selectivity, connected to the secondary coil of theintermediate-frequency transformer 32.

The frequency mixer 31 comprises a first npn transistor Q11 the base ofwhich receives an FM reception signal (high-frequency signal) input, asecond npn transistor Q12 the base of which receives a DC biaspotential, a resistor R connected between the emitters of the npntransistors Q11 and Q12, a first current source Ia connected between theemitter of the first npn transistor Q11 and a ground potential GND, asecond current source Ib connected between the emitter of the npntransistor Q12 and the GND, a third npn transistor Q13 thecollector-emitter path of which is connected between the Vcc powersource terminal 34 and the first npn transistor Q11, a fourth npntransistor Q14 the collector-emitter path of which is connected betweenthe Vcc power source terminal 34 and the second npn transistor Q12, afifth npn transistor Q15 the emitter of which is connected to theemitter of the third npn transistor Q13, the base of which is connectedto the base of the fourth npn transistor Q14, and the collector of whichis connected to the frequency mixing output terminal 35, and a sixth npntransistor Q16 the emitter of which is connected to the emitter of thefourth npn transistor 14, the base of which is connected to the base ofthe third npn transistor Q13, and the collector of which is connected tothe frequency mixing output terminal 35. Local oscillation signal inputsare applied between the bases of the third and fifth npn transistors Q13and Q15 and between the bases of the sixth and fourth npn transistors 16and 14.

In the circuit in FIG. 1, in order to match the secondary coil of theintermediate-frequency transformer 32 with the impedance of the ceramicfilter 33, a turn ratio of the secondary coil to the primary coil of theintermediate-frequency transformer 32 must be decreased. Therefore, alarge intermediate-frequency conversion gain is difficult to obtain.

On the other hand, in a circuit shown in FIG. 2, a frequency mixer 41,an impedance converting emitter follower 42 connected to the output ofthe frequency mixer 41, and a resistor 43 one terminal of which isconnected to the output of the emitter follower 42 are arranged in abipolar integrated circuit 40. Reference numeral 44 denotes a Vcc powersource terminal; 45, a first frequency mixing output terminal connectedto the output terminal of the frequency mixing circuit 41; and 46, asecond frequency mixing output terminal connected to the other terminalof the resistor 43. Reference numeral 47 denotes anintermediate-frequency tuner consisting of an LC tuner for tuning anintermediate-frequency component of 10.7 MHz of the output from thefrequency mixer 41. The tuner 47 is connected between the Vcc powersource terminal 44 and the first frequency mixing output terminal 45outside the integrated circuit 40. Reference numeral 48 denotes aceramic filter, for obtaining desired selectivity, connected to thesecond frequency mixing output terminal 46 outside the integratedcircuit 40.

In the circuit in FIG. 2, since impedance matching between an outputimpedance of the frequency mixing circuit 41 and the ceramic filter 48is performed by the emitter follower 42, a large intermediate-frequencyconversion gain can be obtained. However, since the emitter follower 42has no signal amplitude limiting function, an intermediate-frequencysignal at a level higher than a desired level is supplied to anintermediate-frequency amplifier (not shown) connected to the output ofthe ceramic filter 48.

In a the circuit shown in FIG. 3, a frequency mixer 51, a resistor 52connected to the load and output of the frequency mixer 51, and anintermediate-frequency limiter 53 are arranged in a bipolar integratedcircuit 50. Reference numeral 54 denotes a Vcc power source terminal,and reference numeral 55 denotes a frequency mixing output terminalconnected to the output terminal of the intermediate-frequency limiter53. Reference numeral 56 denotes a ceramic filter, for obtaining desiredselectivity, connected to the frequency mixing output terminal 55outside the integrated circuit 50.

In the circuit in FIG. 3, a signal amplitude limiting function of theintermediate-frequency limiter 53 can prevent supply of anintermediate-frequency signal having a level higher than a necessarylevel to the sequential intermediate-frequency amplifier. However, sincean intermediate-frequency transformer or an intermediate-frequency tuneris not connected to the load of the frequency mixer 51, an unnecessaryfrequency component except for an intermediate-frequency signalgenerated by the frequency mixer 51 cannot be suppressed, therebydegrading spurious features of the circuit.

As described above, in a conventional frequency converter, when anintermediate signal is extracted from an output of a frequency mixerthrough the intermediate-frequency transformer, a largeintermediate-frequency conversion gain is difficult to obtain. When theintermediate-frequency signal is extracted from the output of thefrequency mixer through an emitter follower, an intermediate-frequencysignal having a level higher than a necessary level is supplied to theintermediate-frequency amplifier connected to the following stage. As aresult, when the intermediate-frequency signal is extracted from theoutput of the frequency mixer through an intermediate-frequency limiter,spurious features of the circuit are degraded.

The present invention has been made to solve the above problem, and hasas its object to provide a frequency converter in which a largeintermediate-frequency conversion gain can be obtained, supply of anintermediate-frequency signal having a level higher than a necessarylevel to a sequential intermediate-frequency amplifier can be prevented,a signal of an unnecessary frequency component except for anintermediate-frequency signal generated by the frequency mixer can besufficiently suppressed, and which is free from the degradation ofspurious features.

Disclosure of Invention

According to the present invention, there is provided a frequencyconverter comprising a frequency mixer formed on a semiconductorintegrated circuit and an impedance conversion/signal amplitude limitdifferential amplifier connected to an output of the frequency mixer,wherein an output terminal of the frequency mixer and an output of thedifferential amplifier are connected to first and second frequencymixing output terminals serving as external terminals of thesemiconductor integrated circuit, respectively.

With the above arrangement, since impedance matching between an outputimpedance of the frequency mixer and that of a intermediate-frequencyfilter can be performed by the impedance conversion/signal amplitudelimit differential circuit, a large intermediate-frequency conversiongain can be obtained, and supplying of an intermediate-frequency signalhaving a level higher than a necessary level to a sequentialintermediate-frequency amplifier can be prevented.

In addition, since an intermediate-frequency tuner, is connected to theload of the frequency mixer, the unnecessary frequency component signal,except for an intermediate-frequency signal generated by the frequencymixer, can be sufficiently suppressed, and spurious features of thecircuit are not degraded.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 3 are circuit diagrams showing a conventional frequencyconverter,

FIG. 4 is a circuit diagram showing a frequency converter according toan embodiment of the present invention, and

FIG. 5 is a circuit diagram showing a frequency converter according to amodification of the embodiment in FIG. 4.

BEST MODE OF CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the accompanying drawings.

FIG. 4 shows a frequency converter used for converting an FM receptionsignal into an intermediate-frequency signal in, e.g., an FM receiver.More specifically, reference numeral 11 denotes a frequency mixer formixing the FM reception signal (high-frequency signal) and a localoscillation signal, and reference numeral 12 denotes an impedanceconversion/signal amplitude limit differential amplifier connected tothe output of the frequency mixer 11. The frequency mixer 11 and thedifferential amplifier 12 are formed in a bipolar integrated circuit 10.Reference numeral 13 denotes a Vcc power source terminal; 14, a firstfrequency mixing output terminal connected to the output terminal of thefrequency mixer 11; and reference numeral 15, a second frequency mixingoutput terminal connected to the output terminal of the differentialamplifier 12. The Vcc power source terminal 13 and first and secondfrequency mixing output terminals 14 and 15 are external terminals ofthe integrated circuit 10. Reference numeral 16 denotes anintermediate-frequency tuner consisting of an LC tuner (L is a coil, andC is a capacitor) for tuning an intermediate-frequency component of 10.7MHz of the output from the frequency mixer 11. The tuner 16 is connectedbetween the Vcc power source terminal 13 and the first frequency mixingoutput terminal 14 outside the integrated circuit 10. Reference numeral17 denotes an intermediate-frequency filter, e.g., a ceramic filter (acenter frequency is 10.7 MHz), for obtaining desired selectivity,connected to the second frequency mixing output terminal 15 outside theintegrated circuit 10. An intermediate-frequency amplifier (not shown)is connected to the next stage of the ceramic filter 17.

The arrangement of the frequency mixer 11 is identical to that of thefrequency mixer 31 shown in FIG. 1. The differential amplifier 12comprises a first npn transistor Q1 the base of which is connected tothe output terminal of the frequency mixer 11 in a DC manner, a secondnpn transistor Q2 the emitter of which is connected to the emitter ofthe first npn transistor Q1 in a DC manner and the base of which isconnected to the Vcc power source terminal 13, and a current source Iconnected between a DC connection point of the emitters of the first andsecond npn transistors Q1 and Q2 and a ground potential GND. Thedifferential amplifier 12 is arranged so as to extract a frequencymixing output from at least one collector of the first and second npntransistors Q1 and Q2. In this embodiment, the collector of the firstnpn transistor Q1 is connected to the Vcc power source terminal 13, aload resistor R_(L) is connected between the collector of the second npntransistor Q2 and the Vcc power source terminal 13, and the collector ofthe transistor Q2 is connected to a second frequency mixing outputterminal 15.

Although DC base potential of the second npn transistor Q2 is almostequal to the DC base potential (a Vcc potential is applied through thecoil L of the LC tuner 16) of the first npn transistor Q1, the AC basepotential of the second npn transistor Q2 is grounded. The resistance ofthe load resistor R_(L) is set to be almost equal to the inputresistance of the ceramic filter 17.

In a frequency converter, since impedance matching between the frequencymixer 11 and the ceramic filter 17 connected to the next stage isperformed by the impedance conversion/signal amplitude limitdifferential amplifier 12, a large intermediate-frequency conversiongain can be obtained, supply of an intermediate-frequency signal havinga level higher than a necessary level to the intermediate-frequencyamplifier connected to the following stage can be prevented. In thiscase, the gain of the differential amplifier 12 is determined by acurrent value of the current source I, thereby setting theintermediate-frequency conversion gain.

Since an intermediate tuner 16 is connected to the load of the frequencymixer 11, a signal of an unnecessary frequency component except for anintermediate-frequency signal generated by the frequency mixer 11 can besufficiently suppressed. Therefore, the spurious features of the circuitare not degraded.

The number of frequency mixing output terminals 14 and 15 of theintegrated circuit 10 is larger than that of the conventional circuitsin FIGS. 1 and 3 by one, but is equal to that of the conventionalcircuit in FIG. 2.

FIG. 5 shows a modification of the frequency converter in FIG. 4. InFIG. 5, in order to change the gain of the differential amplifier 12,resistors R1 and R2 are connected to the emitters of the first andsecond npn transistors Q1 and Q2, respectively, to increase the inputimpedance of the differential amplifier 12. In order to increase thedynamic range of the differential amplifier 12, a first emitter followerconsisting of a third npn transistor Q3 and a resistor R3 is connectedto the input of the first npn transistor Q1, and a second emitterfollower consisting of a fourth npn transistor Q4 and a resistor 4 isconnected to the input of the second npn transistor Q2.

According to the frequency converter in FIG. 5, the same effect as inthe frequency converter in FIG. 4 can be obtained. The characteristicsof the frequency converter are improved, and an intermediate-frequencyconversion gain can be easily set.

INDUSTRIAL APPLICABILITY

As described above, according to the frequency converter of the presentinvention, since impedance matching between the output impedance of afrequency mixer and that of an intermediate-frequency filter connectedto a next stage is performed by a differential amplifier connected tothe output of the frequency mixer, a large intermediate-frequencyconversion gain can be obtained, and supply of an intermediate-frequencysignal having a level higher than a necessary level to anintermediate-frequency amplifier connected to a following stage can beprevented. Since an intermediate-frequency tuner is connected to theload of the frequency mixer, a signal of an unnecessary frequencycomponent except for an intermediate-frequency signal generated by thefrequency mixer can be sufficiently suppressed, and the spuriousfeatures of the circuit are not degraded. Therefore, the frequencyconverter can be effectively applied to an FM receiver or the like.

We claim:
 1. A frequency converter comprising:a frequency mixer formixing a high-frequency signal with a local oscillation signal, and foroutputting a signal consisting of a plurality of signal components; atuner for tuning only a signal component having a constant frequencyamong signals output from said frequency mixer; and a differentialamplifier providing an output terminal and having a first bipolartransistor wherein said tuned signal component is input to a base, and acollector is connected to a first power source terminal, a secondbipolar transistor wherein a bas is connected to said first power sourceterminal, the emitter is connected to the emitter of said first bipolartransistor, and a collector is connected to the output terminal; acurrent source for connecting the second power source terminal to aconnection point between an emitter of said first bipolar transistor andan emitter of said second bipolar transistor, and a resistor connectedbetween said first power source and the collector of said second bipolartransistor.
 2. A frequency converter according to claim 1, wherein saidfrequency mixer and said differential amplifier are incorporated as anintegrated circuit version inside a semiconductor substrate, and saidtuner is separate from said semiconductor substrate.
 3. A frequencyconverter according to claim 1, wherein said tuner consists of a circuithaving a coil and a capacitor connected in parallel.
 4. A frequencyconverter according to claim 1, wherein an intermediate-frequency filteris connected to said output terminal.
 5. A frequency convertercharacterized by comprising:a frequency mixer for mixing ahigh-frequency signal with a local oscillation signal, and foroutputting a signal consisting of a plurality of signal components; atuner for tuning only a signal component having a constant frequencyamong output signals of said frequency mixer, and a differentialamplifier providing an output terminal and having a first bipolartransistor wherein said tuned signal component is input to a base, and acollector is connected to a first power source, a second bipolartransistor wherein a base is connected to an emitter of said firstbipolar transistor, and a collector is connected to said first powersource terminal, a third bipolar transistor wherein a base and acollector are connected to said first power source terminal, a fourthbipolar transistor wherein a base is connected to an emitter of saidthird bipolar transistor, and a collector is connected to the outputterminal, a first resistor connected between the emitter of said firstbipolar transistor and a second power source terminal, a second resistorconnected between an emitter of said second bipolar transistor and acurrent source, a third resistor connected between an emitter of saidthird bipolar transistor and said second power source terminal, a fourthresistor connected between an emitter of said fourth bipolar transistorand said current source, and a fifth resistor connected between saidfirst power source terminal and the collector of said fourth bipolartransistor.
 6. A frequency converter according to claim 5, wherein saidfrequency mixer and said differential amplifier comprise an integratedcircuit in a semiconductor substrate, and said tuner is separate fromsaid semiconductor substrate.
 7. A frequency converter according toclaim 5, wherein said tuner consists of a circuit having a coil and acapacitor connected in parallel.
 8. A frequency converter according toclaim 5, wherein an intermediate-frequency filter is connected to saidoutput terminal.